Factor S3 media uploads out to new provider system.

This introduces a new "provider" system which allows core pieces of
code to be swapped out for different hosting setups.
Media Provider: Add "filesystem" provider for local nginx.

This media provider is for simple single-server installations where an
HTTP server running on the same machine handles serving of the static
media files.

An appropriate setup with nginx is now the default in the new install
script.
POST_setappicon: Use VUploadLength.

This endpoint was missed when fixing file uploads after the
request.post -> request.POST deduplication fixup.
/prefs/apps: Make app icon URLs HTTPS friendly.
scraper_q: Gracefully degrade to thumbnail-only when no embedly.

Between the local filesystem media provider and this change, scraper_q
is workable after a default install now.
install-reddit: Ensure consumer-count.d is owned by the right user.